Alejandro Hernández-Martínez

En este estudio se investiga la confiabilidad de trabes de concreto reforzado sometidas a momento flexionante debido a cargas gravitacionales. La resistencia a la compresión simple del concreto se caracteriza probabilísticamente con muestras de un laboratorio localizado en Aguascalientes, México. No existiendo normatividad oficial en dicha ciudad para diseño de elementos de concreto, se consideran dos versiones de las normas de la Ciudad de México para determinar las funciones de estado límite y obtener las confiabilidades mediante metodologías distintas para dos tipos de secciones. Se determinó que la resistencia a compresión simple del concreto sigue una distribución de extremos tipo Weibull. Se concluye que el usar el formato de un solo factor de carga tanto para carga muerta como para carga viva, como en una versión anterior del reglamento de la Ciudad de México, puede conducir a valores no uniformes de confiabilidad para diferentes cocientes entre la carga viva y la carga muerta. Se concluye también que el formato del ACI y de la versión actual de las normas de la Ciudad de México, conducen a niveles mas uniformes del índice de confiabilidad, y se propone un formato que mejora aún mas dicha uniformidad, mediante una relación que es función del cociente entre carga viva y carga muerta. En lo concerniente a una sección “T” investigada, se observó una disminución en la confiabilidad estructural, lo que podría atribuirse al método empleado. Los resultados muestran que la confiabilidad de trabes de concreto podría depender de las características de materiales y resistencias de la región en estudio, e.g. el Bajío, y que distintos formatos de factores de carga y resistencia, así como expresiones para determinar la capacidad a flexión podrían usarse para conocer la confiabilidad implícita de elementos existentes, y para diseñar elementos de concreto nuevos para una confiabilidad objetivo.

Este artículo de investigación aborda un análisis estadístico de aforos vehiculares reales de vialidades situadas en México, a partir de datos obtenidos mediante sistemas de pesaje en movimiento (WIM, por sus siglas en inglés). El objetivo principal de la investigación fue describir fluctuaciones de parámetros del tránsito, como la velocidad, la distancia entre vehículos y la configuración vehicular (clasificación), para obtener una caracterización del comportamiento de vehículos en tramos carreteros. Para ello, se analizaron registros vehiculares provenientes de casos de estudio representativos, correspondientes a distintos periodos y configuraciones viales. En alineación con lo anterior, se desarrollaron códigos de programación en el entorno de MATLAB para el procesamiento de volúmenes de datos, a través los cuales, fue posible filtrar y determinar velocidades promedio, distancias estimadas entre vehículos y patrones de configuraciones vehiculares basados en su correspondiente geometría. Los resultados muestran que existen diferencias significativas en el comportamiento del tránsito según el tipo de vialidad y el año del estudio, con particular énfasis en la dispersión de datos. Asimismo, se evidenció que el mecanismo de clasificación normativa, NOM-012-SCT-2 2017, de las distintas configuraciones vehiculares muestra limitaciones para representar adecuadamente la variabilidad geométrica de los vehículos cuando se disponen de datos de circulación real. En más, los hallazgos derivados de la investigación pretenden contribuir al desarrollo de herramientas de análisis de predicción de daños, mantenimiento y diseño estructural de puentes vehiculares y, en un sentido más general, de la infraestructura vial del país.

In structural and bridge engineering, the axle weights and interaxle spacings of heavy trucks are useful for assessing the capacity of existing bridges, developing live load models, and other issues. Weigh-in-motion data have become the most common source for recording axle weights and interaxle spacings; however, information is not as direct and may not be as precise as that from static surveys. Surveying vehicles by stopping them beside the highway is not common nowadays; nevertheless, surveys provide very reliable information on truck axle weights and interaxle spacing. In this study, data from three surveys on two Mexican highways recorded in 2016 and 2018 are provided. The data contain the gross vehicular weights, axle weights, and interaxle spacings of heavy trucks. The discussion is given as to how the provided information can be useful for the bridge and transportation engineering community and for reliability and code calibration tasks for Mexican bridges and a future design code for bridges in Mexico City. It is concluded that statistical values are consistent with WIM data, with differences due to different methods used, recording time, samples size and others, and that trucks heavier than the legal weight circulate in Mexican highways; static surveys are useful to strongly support this important issue. Further research to compare samples from different surveying techniques, as well as the use of the information to investigate load effects on bridges, is recommended.

In this paper, the efficacy of code-based live-load models is assessed. The paper focuses on their ability to accurately estimate vehicular loads transferred to bridge substructures, including abutments, piers and foundations. Realistic traffic vehicle data are represented using four weigh-in-motion databases, which provide an authentic representation of vehicle information, thus providing a truthful basis for the examination of the bridges studied. The evaluation includes girder bridge models with single span, as well as two-, three- and four-span continuous pinned support. By analysing exceedance rates, the study compares the extreme force values (i.e. maximum and minimum) obtained from vehicles in the databases with those predicted by selected code live-load models. These exceedance rates are presented in spectra format, as a function of the span length. The significant variations observed in the exceedance rates highlight the need for improving existing live-load models to achieve more accurate estimations of the forces transferred to bridge substructures. Such improvements would lead to more uniform reliability levels for any limit state, such as resistance, fatigue, serviceability and cracking.

The present paper assesses the efficacy of code live-load models in accurately estimating the vehicular loads transferred to bridge substructures, such as abutments, piers, and foundations. Realistic traffic vehicle data are represented using four Weigh-in-Motion databases, which provide an authentic representation of vehicle information, thus providing a realistic basis for the examination of the bridges studied. The evaluation includes various bridge models, such as single-span girder bridges and two-, three-, and four-span continuous pinned-support girder bridges. By analyzing exceedance rates, the study compares the extreme force values obtained for vehicles in the databases with those predicted by selected code live-load models. These exceedance rates are presented in spectra format, as a function of the span length. The significant variations observed in the exceedance rates highlight the need for improving existing code live-load models to achieve more accurate estimations of the forces transferred to bridge substructures. Such improvements would lead to more uniform reliability levels for any limit state, such as resistance, fatigue, serviceability, and cracking.

The forces acting on bridge structural elements caused by live loads are computed by live load models defined in design codes. In most cases, such live load models are defined by studies performed on girder bridges, where extreme values of shear forces and bending moment are intended to be predicted. This paper shows that when code live load models are applied to truss bridges, the estimated forces in some structural elements may not be representative of those caused by actual traffic. Three WIM (weigh-in-motion) databases, which are recorded on roads in Mexico, are used as real traffic data. The results suggest that current code live load models are not entirely adequate to estimate forces in structural elements of truss bridges.

The normality polynomial and multi-linear regression approaches are revisited for estimating the reliability index, its precision, and other reliability-related values for coastal and structural engineering applications. In previous studies, neither the error in the reliability estimation is mathematically defined nor the adequacy of varying the tolerance is investigated. This is addressed in the present study. First, sets of given numbers of Monte Carlo simulations are obtained for three limit state functions and probabilities of failure are computed. Then, the normality polynomial approach is applied to each set and mean errors in estimating the reliability index are obtained, together with its associated uncertainty; this is defined mathematically. The data is also used to derive design points and sensitivity factors by multi-linear regression analysis for given tolerances. Results indicate that power laws define the mean error of the reliability index and its standard deviation as a function of the number of simulations for the normality polynomial approach. Results also indicate that the multi-linear regression approach accurately predicts reliability-related values if enough simulations are performed for a given tolerance. It is concluded that the revisited approaches are a valuable option to compute reliability-associated values with reduced simulations, by accepting a quantitative precision level.

The seismic response of tensile membrane structures (TMS) is investigated. To the authors knowledge, this is the first study on TMSs subjected to a seismic record reported in the literature. A type of membrane structures usually considered as a reference in other works are employed in the present study. The selected hyperbolic-paraboloid fabric structures, also referred as hypar TMSs, are subjected to an earthquake accelerogram from a relatively large earthquake recorded at Norcia, Italy. To obtain the TMSs seismic re- sponse, a finite element formulation reported in a previous study, and which accounts for wrinkling phenomena, orthotropic material modeling, and geometrical nonlinearity, is employed. The analyses are performed in two stages; first for the prestressed case and then the seismic loading is added. It is found that the seismic response of TMSs should not be disregarded by designers beforehand, since important increments in the dynamic response of the displacements produce an incremet of around 9% for stresses. However, a very important increment of around 80% for support reaction forces is computed, when compared with the static case. It is also found that the orientation of the frame-supporting structure has a significant impact on the computed seismic reactions.

En este artículo se presenta un modelo de simulación para estimar la resistencia a flexión de elementos de concreto. El modelo de simulación emplea las hipótesis básicas de diseño de acuerdo con las NTC–04 [1], así como con el ACI 318–11 [2] para vigas de sección rectangular con acero de refuerzo solamente en la zona de tensión. La resistencia a la compresión del concreto se integra al modelo como una variable aleatoria con el fin de estimar la incertidumbre en la resistencia a flexión de las vigas. La resistencia a la compresión del concreto se estima a partir de más de 16 mil pruebas con mezclas diseñadas para obtener 200 kg/cm2 (19.6 MPa) y 250 kg/cm2 (24.5 MPa). El principal objetivo de este trabajo es el de evaluar la influencia que tiene la resistencia a la compresión del concreto en la sobrerresistencia estimada a flexión empleando modelos analíticos que se encuentran en los reglamentos de diseño como lo son las NTC–04 y el ACI 318–11.